Clinical Significance of Staphylococcus aureus 
The coagulase-positive species Staphylococcus aureus is well documented as a human opportunistic pathogen. Nosocomial infections caused by S. aureus are a major cause of morbidity and mortality. Some of the most common infections caused by S. aureus involve the skin, and they include furuncles or boils, cellulitis, impetigo, and postoperative wound infections at various sites. Some of the more serious infections produced by S. aureus are bacteremia, pneumonia, osteomyelitis, acute endocarditis, myocarditis, pericarditis, cerebritis, meningitis, scalded skin syndrome, and various abscesses. Food poisoning mediated by staphylococcal enterotoxins is another important syndrome associated with S. aureus. Toxic shock syndrome, a community-acquired disease, has also been attributed to infection or colonization with toxigenic S. aureus (Murray et al. Eds, 1999, Manual of Clinical Microbiology, 7th Ed. ASM Press, Washington, D.C.).
Methicillin-resistant S. aureus (MRSA) emerged in the 1980s as a major clinical and epidemiologic problem in hospitals. MRSA are resistant to all β-lactams including penicillins, cephalosporins, carbapenems, and monobactams, which are the most commonly used antibiotics to cure S. aureus infections. MRSA infections can only be treated with more toxic and more costly antibiotics, which are normally used as the last line of defense. Since MRSA can spread easily from patient to patient via personnel, hospitals over the world are confronted with the problem to control MRSA. Consequently, there is a need to develop rapid and simple screening or diagnostic tests for detection and/or identification of MRSA to reduce its dissemination and improve the diagnosis and treatment of infected patients.
Methicillin resistance in S. aureus is unique in that it is due to acquisition of DNA from other coagulase-negative staphylococci (CNS), coding for a surnumerary β-lactam-resistant penicillin-binding protein (PBP), which takes over the biosynthetic functions of the normal PBPs when the cell is exposed to β-lactam antibiotics. S. aureus normally contains four PBPs, of which PBPs 1, 2 and 3 are essential. The low-affinity PBP in MRSA, termed PBP 2a (or PBP2′), is encoded by the chromosomal mecA gene and functions as a β-lactam-resistant transpeptidase. The mecA gene is absent from methicillin-sensitive S. aureus but is widely distributed among other species of staphylococci and is highly conserved (Ubukata et al., 1990, Antimicrob. Agents Chemother. 34:170-172).
By nucleotide sequence determination of the DNA region surrounding the mecA gene from S. aureus strain N315 (isolated in Japan in 1982), Hiramatsu et al. have found that the mecA gene is carried by a novel genetic element, designated staphylococcal cassette chromosome mec (SCCmec), inserted into the chromosome. SCCmec is a mobile genetic element characterized by the presence of terminal inverted and direct repeats, a set of site-specific recombinase genes (ccrA and ccrB), and the mecA gene complex (Ito et al., 1999, Antimicrob. Agents Chemother. 43:1449-1458; Katayama et al., 2000, Antimicrob. Agents Chemother. 44:1549-1555). The element is precisely excised from the chromosome of S. aureus strain N315 and integrates into a specific S. aureus chromosomal site in the same orientation through the function of a unique set of recombinase genes comprising ccrA and ccrB. Two novel genetic elements that shared similar structural features of SCCmec were found by cloning and sequencing the DNA region surrounding the mecA gene from MRSA strains NCTC 10442 (the first MRSA strain isolated in England in 1961) and 85/2082 (a strain from New Zealand isolated in 1985). The three SCCmec have been designated type I (NCTC 10442), type II (N315) and type III (85/2082) based on the year of isolation of the strains (Ito et al., 2001, Antimicrob. Agents Chemother. 45:1323-1336) (FIG. 1). Hiramatsu et al. have found that the SCCmec DNAs are integrated at a specific site in the methicillin-sensitive S. aureus (MSSA) chromosome. They characterized the nucleotide sequences of the regions around the left and right boundaries of SCCmec DNA (i.e. attL and attR, respectively) as well as those of the regions around the SCCmec DNA integration site (i.e. attBscc which is the bacterial chromosome attachment site for SCCmec DNA). The attBscc site was located at the 3′ end of a novel open reading frame (ORF), orfX. The orfX potentially encodes a 159-amino acid polypeptide sharing identity with some previously identified polypeptides, but of unknown function (Ito et al., 1999, Antimicrob. Agents Chemother. 43:1449-1458). Recently, a new type of SCCmec (type IV) has been described by both Hiramatsu et al. (Ma et al, 2002, Antimicrob. Agents Chemother. 46:1147-1152) and Oliveira et al. (Oliveira et al, 2001, Microb. Drug Resist. 7:349-360). The sequences of the right extremity of the new type IV SCCmec from S. aureus strains CA05 and 8/6-3P published by Hiramatsu et al. (Ma et al., 2002, Antimicrob. Agents Chemother. 46:1147-1152) were nearly identical over 2000 nucleotides to that of type II SCCmec of S. aureus strain N315 (Ito et al., 2001, Antimicrob. Agents Chemother. 45:1323-1336). No sequence at the right extremity of the SCCmec type IV is available from the S. aureus strains HDE288 and PL72 described by Oliveira et al. (Oliveira et al., 2001, Microb. Drug Resist. 7:349-360).
Previous methods used to detect and identify MRSA (Saito et al., 1995, J. Clin. Microbiol. 33:2498-2500; Ubukata et al., 1992, J. Clin. Microbiol. 30:1728-1733; Murakami et al., 1991, J. Clin. Microbiol. 29:2240-2244; Hiramatsu et al., 1992, Microbiol. Immunol. 36:445-453), which are based on the detection of the mecA gene and S. aureus-specific chromosomal sequences, encountered difficulty in discriminating MRSA from methicillin-resistant coagulase-negative staphylococci (CNS) because the mecA gene is widely distributed in both S. aureus and CNS species (Suzuki et al., 1992, Antimicrob. Agents. Chemother. 36:429-434). Hiramatsu et al. (U.S. Pat. No. 6,156,507) have described a PCR assay specific for MRSA by using primers that can specifically hybridize to the right extremities of the 3 types of SCCmec DNAs in combination with a primer specific to the S. aureus chromosome, which corresponds to the nucleotide sequence on the right side of the SCCmec integration site. Since nucleotide sequences surrounding the SCCmec integration site in other staphylococcal species (such as S. epidermidis and S. haemolyticus) are different from those found in S. aureus, this PCR assay was specific for the detection of MRSA. This PCR assay also supplied information for MREP typing (standing for «mec right extremity polymorphism») of SCCmec DNA (Ito et al., 2001, Antimicrob. Agents Chemother. 45:1323-1336; Hiramatsu et al., 1996, J. Infect. Chemother. 2:117-129). This typing method takes advantage of the polymorphism at the right extremity of SCCmec DNAs adjacent to the integration site among the three types of SCCmec. Type III has a unique nucleotide sequence while type II has an insertion of 102 nucleotides to the right terminus of SCCmec type I. The MREP typing method described by Hiramatsu et al. (Ito et al., 2001, Antimicrob. Agents Chemother. 45:1323-1336; Hiramatsu et al., 1996, J. Infect. Chemother. 2:117-129) defines the SCCmec type I as MREP type i, SCCmec type II as MREP type ii and SCCmec type III as MREP type iii. It should be noted that the MREP typing method cannot differentiate the new SCCmec type IV described by Hiramatsu et al. (Ma et al., 2002, Antimicrob. Agents Chemother. 46:1147-1152) from SCCmec type II because these two SCCmec types exhibit the same nucleotide sequence to the right extremity.
The set of primers described by Hiramatsu et al. as being the optimal primer combination (SEQ ID NOs.: 22, 24, 28 in U.S. Pat. No. 6,156,507 corresponding to SEQ ID NOs.: 56, 58 and 60, respectively, in the present invention) have been used in the present invention to test by PCR a variety of MRSA and MSSA strains (FIG. 1 and Table 1). Twenty of the 39 MRSA strains tested were not amplified by the Hiramatsu et al. multiplex PCR assay (Tables 2 and 3). Hiramitsu's method indeed was successful in detecting less than 50% of the tested 39 MRSA strains.
This finding demonstrates that some MRSA strains have sequences at the right extremity of SCCmec-chromosome right extremity junction different from those identified by Hiramatsu et al. Consequently, the system developed by Hiramatsu et al. does not allow the detection of all MRSA. The present invention relates to the generation of SCCmec-chromosome right extremity junction sequence data required to detect more MRSA strains in order to improve the Hiramatsu et al. assay. There is a need for developing more ubiquitous primers and probes for the detection of most MRSA strains around the world.